1. Field of Endeavor
The present invention relates to a gas turbine.
In particular the present invention refers to the sealing of the zone between the guide vane boxes of the high-pressure turbine immediately downstream of the combustion chamber and a fixed frame, such that possible leakages of hot gases flowing in the combustion chamber and/or compressed air used to seal the zone between the combustion chamber and stator airfoil row do not enter the rotor airfoils cooling circuit.
2. Brief Description of the Related Art
In the following reference, will be made to FIG. 1 for describing the relevant parts of the gas turbine; in particular reference will be made to a sequential combustion gas turbine, it is anyhow clear that structures embodying principles of the present invention may be implemented in any gas turbine also not being a sequential combustion gas turbine.
Sequential combustion gas turbines 1 have a compressor (not shown) compressing air and supplying it to first burners (not shown) where fuel is injected and a mixture to be combusted is formed.
Downstream of the first burners a first combustion chamber 2 is provided, where the mixture is combusted to form high pressure hot gases F that are supplied to a high-pressure expansion stage.
The high-pressure expansion stage includes a stator airfoil row 4 separated from the combustion chamber 2 by a first gap 5, and a rotor airfoil row 6 separated from the stator airfoil row 4 by a second gap 7; third gaps 8 are provided between the rotor airfoil row 6 and an annular duct 9 feeding a plurality of side-by-side second burners 10, wherein further fuel is injected in the hot gases (still rich in air) already partially expanded in the high-pressure expansion stage, such that an ignitable mixture is formed. This ignitable mixture is combusted in a second combustion chamber (not shown) and the hot gases produced are further expanded in a low pressure turbine (not shown).
The stator airfoil row 4 is made of stator airfoils 15 defining between each other guide vanes and having endwalls 16 connected to guide vane boxes 17.
The guide vane boxes 17 have a box structure and are fed with cooling air A via connections not shown for simplicity.
In particular, the cooling air A comes from the compressor at a temperature of about 450-550° C. and is cooled by an external cooler to a temperature of typically 200-400° C.
Moreover the guide vane boxes 17 are also provided with nozzles 20 that inject the cooling air A into the second gap 7.
The rotor airfoil row 6 includes a plurality of rotor airfoils 22 having a hollow body provided with an inlet 23 arranged to collect the cooling air A injected from the nozzles 20.
During operation, the hot gases F formed in the first combustion chamber 2 pass through the stator and rotor airfoil row 4, 6 such that the rotor airfoil row 6 extracts mechanical power from them.
Moreover, the air A from the guide vane boxes 17 is injected through the nozzles 20 in the second gap 7 towards the rotor airfoil inlets 23.
As the rotor airfoil row 6 rotate with high speed, it draws the cooling air A injected from the nozzles 20 and makes it to enter the rotor airfoil 22 via the inlets 23.
The cooling air A entering the rotor airfoils 22 cools the rotor airfoils 22 and is then injected through holes (usually at the leading edge and trailing edge of each rotor airfoil row); the air injected through the leading and trailing edges of the rotor airfoils 22 is indicated by A2.
In order to prevent the hot gases F from entering the first gap 5 (the hot gases have a temperature of about 1200-1500° C. and would impair the components close to the first gap 5), compressed air (the so-called ‘purge air’) is diverted from the compressor and is injected in the first gap 5. This air has a temperature of about 450-550° C. and thus is not dangerous for the components close to the gaps 5.
In addition, in order to prevent the compressed air (purge air) from reaching the rotor airfoil inlet 23, seals 25 are provided between the stator airfoil endwalls 16/guide vane boxes 17 and a fixed frame 26.
Nevertheless, the compressed air diverted from the compressor may leak and pass through the seals 25 and mix with the cooling air A injected in the second gap 7.
For this reason, the cooling air A flow rate is quite large, such that, in all operating conditions, the air entering the rotor airfoil 22 has a correct temperature to safeguard the rotor airfoil integrity and guarantee their lifetime.
Nevertheless, since the cooling air A flow rate diverted from the compressor into the guide vane boxes is quite large, efficiency of the gas turbine is reduced.
FR 1 351 268 discloses a guide vane box with openings from which compressed air is injected to enter cooling conduits of the rotor airfoils.
GB 2 246 836 discloses a guide vane with first and second passages; from these passages cooling air is injected into cooling passages of the rotor airfoils. The second passages can be blocked off by a Belleville washer.
EP 0 636 765 discloses guide vane boxes with passages from where a flow is injected into rotor airfoil inlets of a rotor cooling circuit.